This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-235669, filed Aug. 23, 1999, filed the entire contents of which are incorporated herein by reference.
The present invention relates to an image forming method and apparatus, and more specifically, to a technique for controlling the respective surface temperatures of a transfer medium and a carrying body for holding a visible image thereon, thereby improving the transfer efficiency of the visible image.
A wet image forming apparatus, such as an electrophotographic or electrostatic recording apparatus, which uses a liquid developer, has the following advantages over a dry image forming apparatus in the main, and its new merits have recently been being discovered. The liquid developer can ensure high image quality, since it can be formed containing very fine toner particles of a submicron size. Further, satisfactory image density can be obtained with use of a small quantity of toner, so that high economical efficiency and texture of images as fine as that of printed matter (e.g., by offset printing) can be enjoyed. Since the toner can be fixed to paper at a relatively low temperature, furthermore, energy-saving can be realized.
On the other hand, the conventional wet image forming apparatus that uses the liquid developer involves some essential problems, and it has long been subordinate to the dry technique. Transfer means has one of these problems. Lowering of image quality is a first problem on transfer. Conventionally, a developer on a carrying body for holding a latent image thereon is transferred directly to paper by the agency of an electric field of the transfer means. Therefore, the transferred image is subject to unevenness that is attributable to field fluctuation caused by the irregularity of the surface of the paper. Further, transfer failure easily occurs due to variation in the electrical properties of the paper or dependence on the environment.
These problems lower the quality of transferred images considerably. In order to solve the problems, an apparatus is proposed such that a latent image is temporarily transferred from a carrying body for holding it thereon to an intermediate transfer medium, and then transferred to paper. Described in U.S. Pat. Nos. 5,148,222; 5,166,734; and 5,208,637 are apparatuses in which a latent image is transferred to paper by means of pressure (and heat) after it is transferred from a carrying body for holding it thereon to an intermediate transfer medium by means of an electric field.
Described in Jpn. Pat. Appln. KOKOKU Publication No. 46-41679 and Jpn. Pat. Appln. KOKAI Publication No. 62-280882, moreover, are apparatuses in which both image transfer to an intermediate transfer medium and image transfer to paper are carried out by using pressure (and heat) without using an electric field. It is relatively easy to form the intermediate transfer medium from a material that has a smooth surface and is subject to less variation or fluctuation in electrical resistance. Thus, the possibility of lowering of image quality can be made lower than in the case where an image is transferred directly to paper by means of an electric field.
Lowering of image quality is restrained if the image is transferred to the intermediate transfer medium by means of pressure and heat. Since the image is transferred to paper by means of heat and pressure according to this method, the problems that are peculiar to the case of field transfer cannot easily arise. However, these proposed apparatuses are still subject to the following practical problems.
With use of heat and pressure only, it is hard to transfer a toner image of a very low density on a latent image carrying body to the intermediate transfer medium or paper with a high efficiency. In other words, only those portions of an image which have somewhat high density and thick toner layers can be preferentially transferred with ease. Thus, it is hard to reproduce multistage gradation, whereby the image finally transferred to the paper is liable to have poor gradation.
In an image forming apparatus designed to use a liquid developer and transfer an image by means of pressure, as mentioned above, multistage gradation cannot be reproduced with ease because the transfer efficiency for thin toner image portions is low, although the accuracy of the image can be improved.
The present invention has been made in consideration of these circumstances, and its object is to ensure high transfer efficiency for even thin toner image portions, in an image forming apparatus and method for transferring images by utilizing pressure.
According to a first aspect of the present invention, there is provided an image forming apparatus comprising:
a carrying body configured to hold an electrostatic latent image thereon;
a charger configured to charge the carrying body with electricity;
a light source configured to selectively irradiate with light the carrying body charged by means of the charger, thereby forming the latent image on the carrying body;
a developing unit configured to treat the latent image with a liquid developer containing a carrier solution and toner particles, thereby forming a visible image on the carrying body; and
a pressure member configured to press a transfer medium against the carrying body in a transfer position, thereby transferring the visible image from the carrying body to the transfer medium,
wherein T1xe2x89xa6Tg less than T2 is fulfilled where T1 and T2 are surface temperatures of the carrying body and the transfer medium in the transfer position, respectively, and Tg is a glass transition point of the toner particles.
According to a second aspect of the present invention, there is provided an image forming apparatus comprising:
a rotatable photosensitive body configured to hold an electrostatic latent image thereon;
a charger configured to charge the photosensitive body with electricity;
a light source configured to selectively irradiate with light a surface of the photosensitive body charged by means of the charger, thereby forming the latent image on the surface of the photosensitive body;
a developing unit configured to treat the latent image with a liquid developer containing a carrier solution and toner particles, thereby forming a visible image on the surface of the photosensitive body; and
an intermediate transfer body configure to intermediate transfer of the visible image from the surface of the photosensitive body to a record medium, the intermediate transfer body being pressed against the photosensitive body in a transfer position such that the visible image is transferred from the surface of the photosensitive body to a surface of the intermediate transfer body,
wherein T1xe2x89xa6Tg less than T2 is fulfilled where T1 and T2 are surface temperatures of the photosensitive body and the intermediate transfer body in the transfer position, respectively, and Tg is a glass transition point of the toner particles, and a complex coefficient of viscosity of the toner particles at the temperature T2 is at 10,000 poises or more.
According to a third aspect of the present invention, there is provided an image forming method comprising:
charging with electricity a carrying body configured to hold an electrostatic latent image thereon;
selectively irradiating with light the charged carrying body, thereby forming the latent image on the carrying body;
treating the latent image with a liquid developer containing a carrier solution and toner particles, thereby forming a visible image on the carrying body; and
pressing a transfer medium against the carrying body in a transfer position, thereby transferring the visible image from the carrying body to the transfer medium,
wherein T1xe2x89xa6Tg less than T2 is fulfilled where T1 and T2 are surface temperatures of the carrying body and the transfer medium in the transfer position, respectively, and Tg is a glass transition point of the toner particles.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.